Clinical decisions regarding patient management are often made on the basis of blood chemistry analysis. A variety of procedures have been used to perform such analyses, all of which have their deficiencies.
Blood chemistry is often determined on a drawn sample of blood which is subsequently transported to an on-site facility where the analysis is performed. Blood chemistry analysis performed by such a process engenders a risk of contact with the blood sample, an increased risk to the patient of nosocomial infections and the possibility that air emboli may be introduced into the bloodstream, a potential for contamination of the sample, and, perhaps most significantly from the diagnostician's point of view, a lengthy delay between a decision that blood chemistry is necessary and delivery of therapy based on the results of the analysis.
The need for a bedside system to analyze critical blood variables (e.g., O.sub.2, CO.sub.2 and pH) has been addressed by placing environment-sensitive, calibrated optical or electrochemical sensors directly into a patient's artery or vein. Intraarterial or intravenous sensors are typically calibrated by immersion in a solution which has been equilibrated by bubbling with known concentrations of, for example, O.sub.2 and CO.sub.2, to provide a liquid with known partial pressures of O.sub.2 and CO.sub.2 (i.e., pO.sub.2 and pCO.sub.2). The ability of the sensors to detect pO.sub.2 and pCO.sub.2 is then compared with the known pO.sub.2 and pCO.sub.2 .
A major disadvantage of this system is that once a calibrated sensor is inserted into a patient's blood vessel, it must be removed from the vessel for re-calibration and sterilized again before it can be re-inserted. Furthermore, it is equally difficult to perform quality control measurements to determine whether the sensors are functioning properly. Absent the ability to re-calibrate, it is extremely difficult to determine whether the system is performing properly after the sensors have been inserted. In fact, poor performance is frequently seen since (1) intraarterial or intravenous sensors are prone to thrombogenic formations which can cause serious measurement errors and (2) patient movement can result in sensor contact with the vessel wall which can also cause temporary or permanent measurement errors.
An alternative approach is an extracorporeal system or a paracorporeal system for bedside blood chemistry analysis. Extracorporeal systems have been described that house sensors sensitive to parameters in blood. Typical extracorporeal systems are described in U.S. Pat. Nos. 4,791,932 to Margules, 4,989,606 to Gehrich et al., 5,094,820 to Maxwell et al. and 5,165,406 to Wong et al. In these systems, the sensing element is located in or near the wall of the passageway through which the body fluid is passed. Such a configuration may result in inaccurate analyses of blood chemistries due to boundary layer effects. In other words, since the system is typically purged with saline or other infusion medium after each measurements, a boundary layer of medium may remain near the wall of the passageway when blood is subsequently drawn, thereby creating a concentration gradient across the lumen of the passageway. Wong et al. provides an attempt to address this problem by incorporating a helical groove in the passageway to increase turbulence therein.
A paracorporeal system places the sensors in a physiologic line (e.g., arterial or venous line) very near to a patient's arterial catheter. This approach has the primary advantages of eliminating the problems associated with thrombosis and patient movement and, in addition, provides the capability to conduct in situ calibration and quality control checks without compromising sterility. A paracorporeal design allows for a calibration to be made while the sensors are either in the physiologic line (e.g., arterial or venous line) or removed from the physiologic line (i.e., ex vivo). Moreover, quality control checks may be made at any time throughout the life of the sensors. Such a paracorporeal design is disclosed in commonly owned U.S. application Ser. No. 08/379,332, filed Jan. 27,1995, for "In Situ Calibration System for Sensors Located in a Physiologic Line, " by Kimball et al., which is incorporated herein by reference.
There is a need in the art for a cartridge assembly that incorporates sensor module to accurately analyze characteristics in a sample of a physiologic fluid, such as blood, and that is capable not only of monitoring but also regulating the temperature of the sample to enhance the accuracy of the analysis.